The present invention relates to the field of capnography in general, and in particular to instrumentation and methods for respiratory status determination by means of analyzing and interpreting the waveforms obtained in capnographic measurements.
Capnography, as is well known in the art, is the measurement of the level of carbon dioxide (CO2) in exhaled or inhaled breath. The information obtained from a capnographic measurement is generally presented on the display of the capnograph as a series of waveforms, representing the partial pressure of CO2 in the patient""s exhaled breath as a function of time. The information can be displayed at a fast rate, in which case the fine detail of each breath can be evaluated, or at a slow rate, in which case the trend in the end tidal value of exhaled CO2 (EtCO2) can be evaluated.
Clinicians use capnography in order to assess a patient""s respiratory status Respiratory arrest and shunt can be speedily diagnosed, and a whole range of other respiratory problems and conditions can be determined by the shape and trends of the waveforms. In conjunction with other measurements, pulmonary blood flow, embolism and cardiac arrest can be assessed. Capnography is considered to be a prerequisite both for safe intubation and general anesthetic, and for correct ventilation management. Furthermore, recently proposed standards will mandate the use of capnographic monitoring of patients under sedation, even in simple everyday situations such as minor cosmetic surgical procedures, or pediatric dental surgery, or pediatric sedation for the performance of imaging procedures such as MRI.
Up to now, the interpretation of capnographic waveforms has generally been performed by the respiratory specialist or the anesthetist. Such personnel have sufficient experience to be able to perform an accurate diagnosis from an inspection of the waveforms on the monitor screen of the capnograph. However, with the increased use of capnography both by medica specialists in other fields and by paramedical personnel in emergency service environments, the capnograph operator is not always able to make a safe diagnosis of the patient""s respiratory condition by observing the waveform graphs. The waveforms typically have a large number of different parameters, each of which taken by itself can indicate a different condition, and combinations of which can indicate yet further different conditions. These conditions relate not only to the physiological state of the patient, but also to the correct operation and connection of respiratory support apparatus
There therefore exists an important need for instrumentation which will provide diagnostic support for the capnograph operator, beyond the display of the capnographic waveforms and trends on a monitor screen.
The present invention seeks to provide a new capnographic instrument which performs an analysis of the breath waveforms measured by the carbon dioxide sensor, interprets the results of this analysis and outputs to the operator a suggested diagnosis or diagnoses of the respiratory clinical state of the patient, or advises of possible malfunctions of the ventilation equipment being used to assist the patient""s breathing.
There is thus provided in accordance with a preferred embodiment of the present invention, a capnograph, which provides to the operator interpretation of the capnographic waveforms measured, in the form of diagnostic information about the respiratory status of the patient, or about the adequacy of the breathing support provided by the ventilation equipment to which the patient is attached. The instrument provides this information by comparing a number of parameters characteristic of the waveforms of the patient""s breath with an internal library of the values of those parameters expected from normal waveforms stored in its memory. These parameters may either relate to specific features of the waveform in the time domain, or may characterize spectral components of the waveform in the frequency domain. The differences detected are interpreted as an indication of respiratory malfunction in the patient, or as a measure of the effectiveness of the ventilation equipment, or of the patient""s connection thereto. The diagnosis suggested by the system is dependent on which of the particular parameters are deviant from the normal values, and by how much. According to a preferred embodiment of the present invention, several parameters which characterize the form, frequency and magnitude of the waveform are determined for each waveform, and the values input to the processor unit of the instrument for comparison with values of these parameters expected in normal waveforms.
In addition to the analysis of individual waveforms, any long term trend in the values of the parameters is also detected, thus providing an indication of changes occurring in the respiratory state of the patient, whether as a result of his clinical state or of the performance of the ventilation equipment to which he is attached.
There is also provided in accordance with a preferred embodiment of the present invention, a method whereby the analysis and interpretation of capnographic waveforms and waveform trends are performed to provide possible diagnoses of a patient""s clinical respiratory state, or indications of the effectiveness of the patient""s ventilation equipment.
A useful feature of capnographic interpretation would be the determination of the value of the arterial level of carbon dioxide (PaCO2) from the measured capnographic value of EtCO2. It is often assumed that the value of PaCO2 is close to that of EtCO2 or only a few mm. of Hg higher. In effect, this is only so for subjects in good respiratory health, with a normal perfusion/ventilation ratio of pulmonary activity. For subjects with any form of dead space ventilation, or with defective perfusion mechanisms, the two values can be widely different. Since wasted ventilation is so common among a large percentage of respiratory ailments, such as severe respiratory failure, pulmonary hypoperfusion, pulmonary thromboembolism, systemic hypoperfusion and air embolism, the need to be able to determine the true value of PaCO2 from the measured value of EtCO2 is of great importance. Alternative methods of determining PaCO2 involve invasive procedures, such as blood gas analysis.
According to a further preferred embodiment of the present invention, there is thus provided a capnograph, which, as well as measuring the usual carbon dioxide partial pressure. In the patient""s breath, simultaneously measures the partial pressure of oxygen. A high oxygen level in the patient""s exhaled breath is indicative of poor perfusion. According to this embodiment of the present invention, the instrument utilizes this indication of the perfusion efficiency to provide an indication of discrepancy between the values of EtCO2 obtained from the carbon dioxide capnographic values, and the value of the arterial PaCO2 
In addition, to the determination of oxygen utilization, the ventilated volume of gas may be used in order to estimate the discrepancy between the measured values of EtCO2 and the arterial PaCO2 level. A low ventilation volume, which may be indicative of a perfusion shunt condition, causes the measured values of EtCO2 to be less than the values of PaCO2. In accordance with a further preferred embodiment of the present invention, there is therefore provided a capnographic measurement instrument, which incorporates a measurement of the ventilated volume and provides an indication of divergence of the measured values of EtCO2 from the PaCO2 level. The ventilated volume measurement can be obtained from a flow meter sensor section incorporated into the instrument according to the present invention, or, according to a flatter preferred embodiment, it may be obtained from a remote measurement and incorporated into a computation of the EtCO2 levels.
According to yet further preferred embodiments of the present invention, it is also possible to incorporate further diagnostic measurements in the output of the automated capnograph described hereinabove, so as to support the proposed diagnostic interpretations output by the capnograph, or, to provide the operator with more information in order to choose the correct diagnosis, where the capnograph proposes more than one possibility.
According to a further preferred embodiment of the present invention, there is thus provided a capnograph incorporating the ability of self-interpretation of the breath waveforms, which executes these functions by means of the analysis of the waveforms in the frequency domain. Such an instrument is effective in detecting conditions such as the presence of the potentially fatal near zero or zero breathing zones which occur as a result of the slow beat frequency effect observed when a patient is undergoing high frequency ventilation, and the heart rate happens to fall at a frequency close to the HFV frequency.
In addition, when processing is performed in the frequency domain, it is comparatively easy to detect the high frequency components of cardiogenic oscillations in the waveform.
In accordance with yet another preferred embodiment of the present invention, there is provided apparatus for the automatic interpretation of a waveform of a breath of a subject.
In accordance with still another preferred embodiment of the present invention, there is provided apparatus for the diagnosis of the respiratory state of a subject by computer analysis of at least one breath waveform obtained from the subject.
There is further provided in accordance with yet another preferred embodiment of the present invention, apparatus as described above and wherein the waveform is obtained from a capnographic analyzer.
In accordance with a further preferred embodiment of the present invention, there is also provided apparatus for computerized breath analysis consisting of a breath receiver in fluid communication with a subject a breathing analyzer coupled to the breath receiver and operative to analyze breathing of the subject and provide at least one output, and a respiration diagnosis generator providing an indication of the respiratory status of the subject based on at least one output of the breathing analyzer.
There is provided in accordance with yet a further preferred embodiment of the present invention apparatus for computerized breath analysis consisting of a breath receiver in fluid communication with a subject, and a breathing analyzer coupled to the breath receiver and operative to analyze breathing of the subject and to provide an indication of the difference between the end tidal carbon dioxide partial pressure and the arterial carbon dioxide partial pressure of the subject.
There is even further provided in accordance with a preferred embodiment of the present invention apparatus for computerized breath analysis consisting of a breath receiver in fluid communication with a subject, and a breathing analyzer coupled to the breath receiver and operative to analyze breathing of the subject and to provide an indication of the arterial carbon dioxide partial pressure of the subject.
Furthermore, in accordance with yet another preferred embodiment of the present invention, there is provided apparatus for computerized breath analysis as described above and also consisting of a respiration diagnosis generator providing an indication of the respiratory status of the subject based on the indication of the difference between end tidal carbon dioxide partial pressure and the arterial carbon dioxide partial pressure of the subject,
There is also provided in accordance with a further preferred embodiment of the present invention apparatus for computerized breath analysis as described above and also consisting of a respiration diagnosis generator providing an indication of the respiratory status of the subject based on the indication of the arterial carbon dioxide partial pressure of the subject.
In accordance with yet another preferred embodiment of the present invention, there is provided apparatus as described above and wherein the breathing analyzer includes a capnograph.
There is further provided in accordance wit yet another preferred embodiment of the present invention apparatus as described above and wherein the breathing analyzer performs analysis of at least one breath waveform.
In accordance with still another preferred embodiment of the present invention, there is provided apparatus as described above and wherein the breathing analyzer operates in a frequency domain.
There is further provided in accordance with still another preferred embodiment of the present invention apparatus as described above and wherein the breathing analyzer analyzes changes in the frequency spectrum of the breath waveform for detecting changes in the respiratory status of the subject.
In accordance with a further preferred embodiment of the present invention, there is also provided apparatus as described above and wherein the breathing analyzer compares at least part of the breath waveform with at least one reference.
There is provided in accordance with yet a further preferred embodiment of the present invention apparatus as described above and wherein the breathing analyzer analyzes variations in at least part of the breath waveform over time for detecting changes in the respiratory status of the subject.
There is even further provided in accordance with a preferred embodiment of the present invention apparatus as described above and wherein the respiratory diagnosis generator provides an indication of the respiratory status of the subject based on a capnographic measurement and also on at least one output of at least one of a breathing flow rate measurement, a breathing volume measurement, an analysis of the amounts of at least one other gas in the breath, a pulse oximetry measurement of the pulse rate and/or of the oxygen saturation level SpO2, a measurement of cardiac output, of body temperature, and an ECG measurement.
Furthermore, in accordance with yet another preferred embodiment of the present invention, there is provided apparatus for providing an indication of the respiratory state of a subject, consisting of a gas analyzer which provides at least one time-dependent waveform of the partial pressure of carbon dioxide in the breath of a subject, an input circuit for receiving a sequence of digitized signals derived from the at least one waveform, a signal processor for analyzing the at least one waveform for at least one parameter which characterizes a property of the waveform, a parameter comparator for comparing the at least one parameter with a predefined parameter which characterizes the same property of a waveform obtained from a normal subject, and a diagnosis generator which generates a message in accordance with the output of the parameter comparator.
In accordance with yet another preferred embodiment of the present invention, there is provided apparatus for providing an indication of the respiratory state of a subject, consisting of a gas analyzer which provides a plurality of time-sequenced waveforms of the partial pressure of carbon dioxide in the breath of a subject, an input circuit for receiving a sequence of digitized signals derived from the plurality of time-sequenced waveforms, a signal processor for analyzing the plurality of time-sequenced waveforms for determining changes with time in at least one parameter of the plurality of time-sequenced waveforms, a trend analyzing unit for comparing the changes with time in the at least one parameter of the plurality of time-sequenced waveforms, with predefined changes with time in the equivalent the at least one parameter of time-sequenced waveforms typical of patients with known respiratory deficiencies, and a diagnosis generator which generates a message in accordance with the output of the trend analyzing unit.
There is further provided in accordance with yet another preferred embodiment of the present invention apparatus as described above and wherein the respiratory deficiencies arise from deficient ventilation.
There is also provided in accordance with a further preferred embodiment of the present invention apparatus as described anywhere above and wherein the gas analyzer is a capnograph.